A number of devices are commercially available which a user conducts a resistance exercise by lifting weights. The weights can be connected to a bar and freely lifted by the user or housed in a frame and coupled to the user in a great number of fashions in order to exercise a desired muscle. Movement of the weights is determined by the input force exerted by the user. Examples of this type of exercise device is shown in U.S. Pat. No. 3,235,255, Laflar, and U.S. Pat. No. 3,346,256, White.
The White and the Laflar devices use weights to exert a load on the user during the exercise motion. The force exerted by the user is a function of mass of the weight and the rate of acceleration. The maximum weight capable of being lifted by the user is determined by the portion of the exercise movement where the user is the weakest; this is typically the muscle shortening or positive movement. During any bodily movement some muscles will be contracting while others will be extending. In an exercise movement, the muscles or muscle group, which are doing the majority of the work is typically called the prime mover. When the prime mover muscles are being shortened, it is typically referred to as a positive exercise movement. When the prime mover muscles are lengthening this is typically referred to as a negative movement. In the muscle lengthening, or negative interval of the exercise movement, when the user is capable of carrying more weight, the White and Laflar devices make no provisions to fully exercise this unused muscular capacity.
Realizing that the muscle is not equally powerful throughout its entire range, and that the resistive force should vary as a function of the output force capable of the user, the device shown in U.S. Pat. No. 3,998,100, Pizatella et al, was developed. The object of the Pizatella invention is to exert a force on the user during a repetitive exercise motion. If the user is unable to maintain the predetermined rate of speed, the opposed resistive force is automatically decreased, so that the user has sufficient strength to keep pace with the programmed exercise speed. In this manner the device can fully tax the muscular potential of the user at all positions during the movement. The device emits a disciplinary conditioning signal to encourage the user to maintain the load and speed programmed for the exercise. The load is exerted upon the user by hydraulic cylinders, the pressure in which is controlled to achieve the desired load.
Another type device in which a user may utilize the full muscular potential at all positions during an exercise motion is the isokinetic apparatus which is shown in U.S. Pat. No. 3,465,592, Perrine. While the user goes through an exercise movement of a predetermined range and speed, he may exert his maximum force upon the apparatus and the movement will not exceed the predetermined rate. On the other hand, the user can exert the minimal effort upon the device and similarly maintain the predetermined rate of movement.